The present invention relates to electronic component mounting methods, and more specifically relates to an electronic component mounting method used for manufacturing a surface-mounted unit in which a surface-mount component and an IC are disposed on a single wiring substrate.
A first example of the background art of the present invention will be described below. In a manufacturing process of a surface-mounted unit (sub-module unit) in which a surface-mount component and an IC are disposed on a single wiring substrate, the surface-mount component and the IC (also referred to simply as electronic components) are mounted on the wiring substrate by, for example, the following method. An IC provided with bumps formed of a Pb-based, high-temperature solder (for example, Pb95/Sn5, melting point: 315xc2x0 C.) is prepared, and is connected and fixed to a predetermined electrode formed on a wiring substrate. In addition, a surface-mount component is connected and fixed to a predetermined electrode formed on the wiring substrate using a eutectic solder (for example, Sn63/Pb37, melting point: 183xc2x0 C.).
According to this method, the IC and the surface-mount component are mounted on the wiring substrate by the following processes.
(1) Flux is first applied to a substrate forming the IC (IC substrate). Then, the IC provided with bumps is disposed on the wiring substrate and a reflow process (peak temperature: 355xc2x0 C.) is performed.
(2) The wiring substrate is cleaned, and a eutectic solder paste for mounting the surface-mount component is supplied to an electrode for receiving the surface-mount component using a dispenser, etc. Then, the surface-mount component is disposed on the wiring substrate and a reflow process (peak temperature: 220xc2x0 C.) is performed.
(3) The wiring substrate is cleaned again, and an underfill resin is injected into the space between the bottom surface of the IC and the wiring substrate and is cured.
Thus, a surface-mounted unit (sub-module unit) in which a surface-mount component and an IC are disposed on a single wiring substrate is completed.
Next, a second example of a known method for mounting a surface-mount component and an IC on a wiring substrate will be described below. In this method, the surface-mount component and the IC are both connected and fixed to electrodes formed on the wiring substrate using a eutectic solder (for example, Sn63/Pb37, melting point: 183xc2x0 C.).
According to this method, the IC and the surface-mount component are mounted on the wiring substrate by the following processes.
(1) A eutectic solder paste for mounting the surface-mount component is supplied to an electrode for receiving the surface-mount component by, for example, a printing process using a metal mask.
(2) The surface-mount component is disposed on the electrode formed on the wiring substrate.
(3) Flux is applied to the IC provided with bumps of eutectic solder.
(4) The IC provided with bumps of eutectic solder is disposed on an electrode for receiving the IC.
(5) The wiring substrate on which the IC and the surface-mount component are disposed is subjected to a reflow process (peak temperature: 220xc2x0 C.).
(6) The wiring substrate is cleaned, and an underfill resin is injected into the space between the bottom surface of the IC and the wiring substrate and is cured.
Thus, a surface-mounted unit (sub-module unit) in which a surface-mount component and an IC are disposed on a single wiring substrate is completed.
However, the above-described first example has the following problems.
(a) Since two reflow processes and two cleaning processes are performed, a long processing time is required. In addition, since two kinds of solders having different melting points are used, two reflow furnaces are required. Thus, production efficiency is low and high equipment costs are incurred.
(b) When the solder bumps on the IC are formed of a Pb-based, high-temperature solder (for example, Pb95/Sn5, melting point: 315xc2x0 C.), the IC must first be mounted and fixed from the viewpoint of the heat resistance of the surface-mount component. Thus, a typical printing process cannot be used for supplying the solder paste for mounting the surface-mount component, and the efficiency is thereby reduced.
(c) The Pb-based, high-temperature solder has a low solder wettability, and self-alignment cannot be ensured unless the reflow process is performed in a reducing atmosphere or flux having a high degree of activity is used.
(d) When the surface-mounted unit (sub-module unit) including the IC and the surface-mount component is mounted on a motherboard, there is a risk that the eutectic solder connecting the surface-mount component will re-melt and the surface-mount component will be displaced or become separated. Thus, reliability cannot be ensured.
(e) With reference to FIG. 15, for example, an electrode 52 is formed on a wiring substrate 51, and a solder resist 56 is applied to the electrode 52 at a region around a solder bump 54 which is formed on an IC 53 and which is connected and fixed to the electrode 52. In the case in which a reflow process is performed after the solder resist 56 is applied, a resist based on resin such as epoxy resin, epoxy acrylate resin, etc., is normally used as the solder resist 56. However, a resin-based solder resist has a low heat resistance, and the adherence to the electrode 52 is reduced because of the heat applied in the reflow process. Thus, solder 54a, which is the solder bump 54 in a molten state, flows under the resist 56, and the height of the solder bump 54 becomes insufficient (the solder bump 54 becomes shorter). As a result, sufficient cleaning cannot be performed in the subsequent process and the underfill resin (not shown) cannot be easily injected into the space between the bottom surface of the IC 53 and the wiring substrate 51. Accordingly, there is a problem in that the connection reliability is reduced.
(f) There is a risk in that solder balls generated in the second reflow process cannot be removed completely in the cleaning process. In such a case, the solder balls may travel under the bottom surface of the IC and obstruct the process of applying (injecting) the underfill resin.
(g) When the Pb-based, high-temperature solder (for example, Pb95/Sn5, melting point: 315xc2x0 C.) is used, the amount of Pb, which causes pollution, is increased. Thus, preferably, the Pb-based, high-temperature solder is not used from the viewpoint of environment protection.
In addition, with regard to the above-described second example, there is a risk in that, when the surface-mounted unit (sub-module unit) including the IC and the surface-mount component is mounted on a motherboard, the eutectic solder connecting the IC and the surface-mount component will re-melt and the surface-mount component will be displaced or become separated. Thus, there is a problem in that reliability cannot be ensured. In the above-described case, the IC will not be displaced or become separated since it is retained by the underfill resin.
In order to solve the above-described problems, an object of the present invention is to provide an electronic component mounting method by which a surface-mount component and an IC can be efficiently and reliably mounted on a single substrate.
In order to attain the above-described object, according to the present invention, an electronic component mounting method for manufacturing a surface-mounted unit in which a surface-mount component and an IC are disposed on a single wiring substrate, comprises the steps of:
(a) supplying a high-temperature solder paste on an electrode for receiving the surface-mount component, the electrode being formed on the wiring substrate;
(b) provisionally fixing the surface-mount component on the electrode with the high-temperature solder paste;
(c) applying flux to an electrode for receiving the IC, the electrode being formed on the wiring substrate and the IC being provided with eutectic solder bumps, or applying the flux to the eutectic solder bumps;
(d) provisionally fixing the IC provided with the eutectic solder bumps on the electrode with the flux; and
(e) reflow-soldering the surface-mount component and the IC by heating the wiring substrate to a high temperature at which both the high-temperature solder and the eutectic solder melt.
The electronic component mounting method includes the above-described steps (a) to (e). The high-temperature solder paste is supplied to the electrode for receiving the surface-mount component, and the surface-mount component is provisionally fixed on the electrode with the high-temperature solder paste. In addition, the flux is applied to the electrode for receiving the IC that is provided with the eutectic solder bumps, or it is applied to the eutectic solder bumps, and the IC is provisionally fixed on the electrode with the flux. Then, the wiring substrate is heated to a high temperature at which both the high-temperature solder and the eutectic solder melt, so that the surface-mount component and the IC are reflow-soldered. Accordingly, both the surface-mount component and the IC are connected in a single reflow process, so that the manufacturing process can be made simpler.
In addition, since the IC is mounted after the high-temperature paste is supplied, the high-temperature paste can be supplied to the electrode for receiving the surface-mount component by a known printing process. In addition, since both the surface-mount component and the IC are connected in a single reflow process, failure when mounting the IC because of the solder balls which are generated in the reflow process for the surface-mount component can be prevented.
In addition, since a eutectic solder (for example, Sn63/Pb37) is used as the solder for connecting the IC, the solder wettability can be improved and the connection reliability can be increased.
In addition, according to the present invention, an electronic component mounting method for manufacturing a surface-mounted unit in which a surface-mount component and an IC are disposed on a single wiring substrate provided with an electrode for receiving the surface-mount component, an electrode for receiving the IC, and eutectic solder bumps formed on the electrode for receiving the IC, comprising the steps of:
(a) supplying a high-temperature solder paste on the electrode for receiving the surface-mount component, the electrode being formed on the wiring substrate;
(b) provisionally fixing the surface-mount component on the electrode with the high-temperature solder paste;
(c) applying flux to an electrode of the IC, or applying the flux to the eutectic solder bumps formed on the electrode for receiving the IC;
(d) provisionally fixing the IC on the electrode with the flux; and
(e) reflow-soldering the surface-mount component and the IC by heating the wiring substrate to a high temperature at which both the high-temperature solder and the eutectic solder melt.
The above-described electronic component mounting method includes the above-described steps (a) to (e). First, the high-temperature solder paste is supplied to the electrode for receiving the surface-mount component, and the eutectic solder bumps are formed on the electrode for receiving the IC (the order of the step of supplying the high-temperature solder paste and the step of forming the eutectic solder bumps is not limited). Then, the surface-mount component is provisionally fixed on the electrode with the high-temperature solder paste, and the flux is applied to the electrode of the IC or to the eutectic solder bumps. Then, the IC is disposed on the electrode and is provisionally fixed on the electrode with the flux. Then, the wiring substrate is heated to a high temperature at which both the high-temperature solder and the eutectic solder melt, so that the surface-mount component and the IC are simultaneously reflow-soldered. Accordingly, both the surface-mount component and the IC are connected in a single reflow process, so that the manufacturing process can be made simpler.
In addition, the high-temperature paste can be supplied to the electrode for receiving the surface-mount component by a known printing process. Furthermore, since both the surface-mount component and the IC are connected in a single reflow process, failure when mounting the IC because of the solder balls which are generated in the reflow process for the surface-mount component can be prevented.
In addition, since a eutectic solder (for example, Sn63/Pb37) is used as the solder for connecting the IC, the solder wettability can be improved and the connection reliability can be increased. Especially for the IC in which the connection area with the solder is smaller than that of the surface-mount component, it is important to use a eutectic solder which has a high connection reliability.
In addition, according to the electronic component mounting method of the present invention, the wiring substrate is a ceramic substrate formed of a ceramic material, and, in the case in which a solder resist is applied to the electrodes at regions around solders, the solder resist is formed of a heat-resistant, insulating material which is not degraded in the reflow-soldering process performed at a high temperature.
Accordingly, a ceramic substrate is used as the wiring substrate, and the solder resist is formed of a heat-resistant, insulating material which is not degraded in the reflow-soldering process performed at a high temperature. In this case, even when the wiring substrate is heated to a high temperature so that the high-temperature solder melts, deterioration and deformation of the wiring substrate can be suppressed compared with the case in which a resin substrate is used. In addition, since the solder resist is formed of a heat resistant material, a situation wherein the solder flows on the surface of the electrode to the region under the resist, thus reducing the height of the bump, can be prevented. Accordingly, the present invention can be more effectively applied.
In addition, according to the electronic component mounting method of the present invention, the solder resist formed of the heat-resistant, insulating material comprises an inorganic insulating material such as ceramic or glass.
An inorganic insulating material such as ceramic or glass is stable under high temperatures and has a high heat resistance. Thus, in the case in which the solder resist formed of an inorganic insulating material is used, a situation wherein the solder flows on the surface of the electrode to the region under the resist, thus reducing the height of the bump, can be prevented. Accordingly, the present invention can be more effectively applied.
In addition, according to the electronic component mounting method of the present invention, after the reflow-soldering process, an underfill resin is injected into the space between the bottom surface of the IC and the wiring substrate.
In the case in which the underfill resin is injected into the space between the bottom surface of the IC and the wiring substrate, even when a reflow process is performed again for mounting the surface-mounted unit on a motherboard, the IC can be reliably prevented from being displaced or separated.
In addition, according to the electronic component mounting method of the present invention, the wiring substrate is provided with an external connection terminal.
The present invention may also be effectively applied in the case in which the wiring substrate is provided with external connection terminals such as BGA terminals, LGA terminals, etc., that is, in the case in which the surface-mounted unit (sub-module unit) is mounted on a motherboard by a user. In such a case, by using the underfill resin, the IC can be prevented from being displaced even when a reflow process is performed again for mounting the surface-mounted unit on a motherboard.
According to the electronic component mounting method of the present invention, the high-temperature solder is a Sn-based, high-temperature solder whose liquidus line temperature is in the range of 210xc2x0 C. to 260xc2x0 C.
By using the Sn-based, high-temperature solder whose liquidus line temperature is in the range of 210xc2x0 C. to 260xc2x0 C., the surface-mount component can be reliably connected and fixed to the electrode formed on the wiring substrate with solder which has a sufficiently high melting point and which does not contain Pb, which causes pollution.